The subject matter of the present invention relates generally to telescopic sights having reticle sight centering means, and in particular, to such telescopic sight having a reticle centering sleeve for maintaining the reticle sight point centered on the optical axis of the erector lenses even when such lenses are adjusted longitudinally to vary the magnification of the telescopic sight. The telescopic sight of the present invention may be used as a rifle scope or for an optical instrument, such as a surveying instrument.
It has previously been proposed in fixed magnification telescopic sights, such as that shown in U.S. Pat. No. 2,858,732 of Kollmorgen et al issued Nov. 4, 1958 and U.S. Pat. No. 3,297,389 of Gibson issued Jan. 10, 1967 to mount the reticle sight by threadedly attaching the reticle mounting ring within a lens holder tube or within a housing tube containing the lens holder tube. In the Kollmorgen patent, the reticle sight moves with the lens holder tube to make adjustments for changes in elevation and windage, while in the Gibson patent the reticle sight is fixed and separate from the lens holder tube so that it does not move with such tube. In both of these prior patents the reticle sight point formed by two crossing wires, may be mounted at a distance slightly displaced from the optical axis of the lens system due to tolerances in the diameter of the reticle mounting ring and housing tube or lens holder tube. However, since these rifle scopes are of a fixed magnification, this lack of centering of the reticle crossing point of the optical axis is not apparent because the image transmitted through the lens system does not move relative to such crossing point due to any change of magnification.
However, if such conventional reticle mounting apparatus is employed in a scope of variable magnification, such as U.S. Pat. No. 3,161,716 of Burris et al issued Dec. 15, 1964, a serious problem is created because when it is off-center from the optical axis the reticle sight point moves relative to the image transmitted through the lens system as the magnification of such image changes. This problem is discussed in the Burris patent and is sometimes referred to as "image drift" since the reticle sight point appears to drift across the image of the target transmitted through the lens system of the scope when the magnification of such lens system is changed. In Burris the reticle mounting ring is only held by longitudinal pressure of a lock ring in a centered position so that vibration and shock due to rifle recoil can loosen and move such mounting ring off-center.
The above-discussed image drift problem is solved by the reticle centering sleeve provided around the reticle mounting ring in the telescopic sight apparatus of the present invention. The reticle centering sleeve is of a tapered cross-section and is positioned around one end of the reticle mounting ring so that it is wedged between the housing tube and the reticle mounting ring to automatically center the sight point on the optical axis of the erector lenses. The reticle centering sleeve is provided with a plurality of inward projections spaced around the circumference of such sleeve, with each projection having a substantially uniform height. Since the reticle centering sleeve is made of synthetic plastic material, such as nylon, the inward projections deform under compression between the lens tube and the mounting ring. As a result, the reticle centering sleeve fills the entire space between the housing tube and the reticle mounting ring to compensate for different diameter tolerances and to prevent loosening of such ring due to vibration and shock. Thus, the centering sleeve centers the reticle sight point on the optical axis of the lenses and maintains such centered position under operating conditions in spite of repeated shock vibrations caused by rifle recoil. The reticle centering sleeve of the present invention provides an inexpensive solution to the image drift problem and, in addition, reduces the radial compression applied to the reticle mounting ring which tends to produce slack in the reticle wires thereby maintaining such reticle wires in a taut condition.